This invention relates to the use of a reverse osmosis system as a preliminary and as a final processing method to obtain ultrapure deionized (DI) water for use in environments where such water is essential, as in the manufacture of integrated circuit (IC) chips.
The process of manufacturing IC chips consists of multiple repetitions of the steps of coating, etching and cleaning semiconductor chips or dies. One of the critical steps in this sequence is the cleaning process following chemical etching. The cleaning process includes loading the wafers into a wafer carrier, placing the wafer carrier in a spin rinser/dryer, and while spinning, rinsing the wafers in deionized water and drying the wafers by passing an inert heated gas over the wafers. The cleanliness of the water is of paramount importance to producing acceptable products. The cleanliness can be monitored during the rinse cycle by a resistivity measuring means that prevents the rinser/dryer from going into a dry cycle until the used rinse water reaches a predetermined resistivity, indicating a proper water cleanliness after rinsing. As much as 2,000 gallons of water may be used to process one loaded wafer carrier, since the process may require 9-12 rinses. The wafer manufacturing and cleaning processes are generally carried out in clean rooms which often exceed the cleanliness requirements of a hospital operating room insuring that wafers do not become contaminated by particulate matter. Consequently, great emphasis is placed on using and reclaiming ultrapure DI water for the washing process. The DI water currently used in the industry for this process often approaches 18 megohms resistivity (the longitudinal electrical resistance of a uniform rod of unit length and unit cross-sectional area), or specific resistance. A typical washing process in the IC manufacturing industry is terminated when the rinse water resistivity approaches 18 megohms (normal household tap water is about 0.003 megohms).
Resistivity is one measure of the quality of DI water. Total organic carbon (TOC) and particulate counts are additional measures indicating water cleanliness or quality. Membrane filters have been developed to reduce the particulate in the water to a level of less than 30 counts per milliliter sizes 0.1 micron to 1 micron, and control of TOC is a major concern in ultrapure water production. Equipment for the reduction or the removal of TOC is a major part of current ultrapure water systems. Currently available TOC removal systems use 185 nanometer ultraviolet (UV) light, or add ozone to the water to reduce TOC levels. Ozone is effective in removing TOC but presents new problems in a recirculating system, since the ozone must be removed from the water before passing through the DI beds to avoid costly damage to the resin.
Finally, reverse osmosis (RO) is a method of manufacturing ultrapure water which has been incorporated into a number of industrial processes. U.S. Pat. No. 4,342,651, issued Aug. 3, 1982, discloses a process to make "sterilized and bacteria-free water suitable for medical injections and other [technical] purposes". The system consists of a DI water source that is passed through a pair of reverse osmosis filters to the final product use. Before use the water may be recycled to improve the cleanliness of the water.
U.S. Pat. No. 4,156,621, issued May 29, 1979, discloses a dishwasher water recovery system using reverse osmosis membranes as the final polishing unit prior to storing and re-using the water. The process uses water pressures of 100-450 psi and temperatures of 70.degree.-80.degree. F., resulting in removal of the 90-95% of "ionized dissolved mineral salts" and removal of 99% of the "non-ionized organic compounds".
Additionally, disclosures contained in the American Institute of Chemical Engineers and the EPA Technology Transfer Proceedings of 1975 relating to reverse osmosis for purifying liquid streams, includes an article entitled "Renovation of a Hospital-Type Wastewater for Recycle", and discloses data relating to removal of TOC and conductivity conditioning RO. The resistivity of the RO water produced therein is 1/148 micromhos, or 0.0067 megohms. The article discloses a 74% average TOC reduction rate.